1. Field of the Invention
The present invention relates to an optical direct amplifier and more particularly, to an optical direct amplifier preferably used for Wavelength Division Multiplexing (WDM) optical transmission.
2. Description of the Related Art
In recent years, with the WDM optical transmission system, electric power consumption has been increasing due to the performance enhancement of optical components or devices such as optical cross connects, the speedup in the operation of optical transmitters/receivers for aiming at realization of the transmission of optical signals at 40 Gbit/s, and so on. For this reason, there is an anxiety that power consumption of the individual optical devices used in the WDM optical transmission system is excessively large. Moreover, there has been the demand of power consumption lowering that includes the consideration for environmental measures as well. In this way, the importance of power consumption lowering has been rising more and more in the WDM optical transmission system.
When an optical signal is transmitted with an optical direct amplifier in the WDM optical transmission system using optical repeaters, there is a possibility that the transmission characteristics deteriorates (i.e., digital errors occur) due to gain deviation within the amplification band of the optical direct amplifier used. This is because the wavelength characteristic of the gain of a rare-earth-doped optical fiber, which is used as an optical amplification medium of the optical direct amplifier, has temperature dependence, and the said temperature dependence of the gain is increased by the ambient temperature change, resulting in the transmission characteristics deterioration. Accordingly, there is the need that the optical amplification medium needs to be kept at a temperature higher than its permissible temperature range to avoid the gain deviation among the channels that is induced by the wavelength characteristic change of the gain. To fulfill this need, conventionally, an optical direct amplifier shown in FIG. 1 has been used.
FIG. 1 is a perspective view of a prior-art optical direct amplifier 110 having a conventional typical structure, showing schematically the structure of the amplifier 110. With this amplifier 110, an optical amplification medium 111 is heated by a heating medium 112 mounted on one side of the amplifier 110.
The optical amplification medium 111 is formed by a reel 111a and an optical fiber 111b for amplification placed around the reel 111a. A temperature sensor 113 is mounted in the vicinity of the optical fiber 111b. A heating medium 112 is an electric heater that generates heat responsive to the supply of electric power. A temperature controller circuit 114 is electrically connected to the heating medium 112 and the optical amplification medium 111 by way of cables 116 and 117, respectively. The temperature controller circuit 114, which monitors the temperature of the optical amplification medium 111 at all times, controls the heating medium 112 in such a way that the temperature of the optical amplification medium 111 is kept at a predetermined temperature higher than the ambient temperature.
Since the prior-art optical direct amplifier 110 has the above-described structure, the temperature of the optical amplification medium 111 (i.e., the optical fiber 111b) whose gain has temperature dependence can be kept constant. Therefore, the effect by the temperature deviation of the gain of the optical fiber 111b is suppressed and as a result, stable WDM optical transmission characteristics that are not affected by the ambient temperature can be obtained. In other words, the temperature dependence of the gain-wavelength characteristics of the optical amplification medium 111 can be suppressed, thereby generating the gain-wavelength characteristics regardless of the ambient temperature.
In addition, the heating medium 112 is provided on only one side of the optical amplification medium 111 (i.e., the reel 111a) in the structure of FIG. 1. However, the heating medium 112 may be provided on each side of the optical amplification medium 111 (i.e., the reel 111a) in the structure of FIG. 1.
The followings are other related art to the present invention than the above-described prior-art optical direct amplifier 110.
The Patent Document 1 (the Japanese Non-Examined Patent Publication No. 2001-7428) published in 2001 discloses an optical amplifier that makes it possible to maintain the gain flatness easily. This optical amplifier is used for WDM optical transmission system.
The optical amplifier of the Patent Document 1 comprises an optical waveguide for optically amplifying a signal light, to which a fluorescent material that can be excited by excitation light is doped (e.g., an optical fiber for amplification to which a fluorescent material is doped); and exciting means for supplying excitation light to the optical waveguide. This optical amplifier further comprises output controlling means for controlling the output in such a way that the power of signal light outputted from the optical waveguide is kept at a target value, and temperature controlling means for controlling the temperature of at least part of the optical waveguide based on the power of the signal light inputted into the optical waveguide.
Concretely speaking, the optical fiber for amplification is placed around a coil bobbin made of a material having excellent thermal conductivity (e.g., aluminum). A Peltier element for adjusting the temperature of the optical fiber and a thermistor for detecting the temperature of the optical fiber are adhered to the coil bobbin. For this reason, if the power of the output signal light seeks to change in accordance with the fluctuation of the power of the inputted signal light, the temperature of the optical fiber is controlled in such a way that the power of the output signal light is kept at the target value. Therefore, the degradation of the optical amplification performance is suppressed even if the power of the inputted signal light fluctuates. As a result, the flatness of gain can be maintained easily. (See claim 1, FIG. 1, and paragraphs 0018 to 0022 of the Patent Document 1.)
The Patent Document 2 (the Japanese Non-Examined Patent Publication No. 8-173560) published in 1996 discloses a laser therapy instrument and a probe used therefor, which make it possible to conduct a laser therapy and a thermotherapy simultaneously for chronic pain treatment without causing any thermal damage to the semiconductor laser.
The probe for the laser therapy instrument disclosed by the Patent Document 2 comprises a semiconductor laser, a thermo-module (e.g., Peltier element) for cooling the semiconductor laser, a thermistor for detecting the temperature of the thermo-module, touch-sensor electrodes for detecting the contact with a human body, a ceramic that emits far-infrared rays, and a probe case for enclosing these components. The thermo-module has an opening at its central position, into which the irradiation mouth of the semiconductor laser is inserted. The heat radiating plate of the semiconductor laser is fixed on the cooling side of the thermo-module. The ceramic for emitting far-infrared rays is fixed on the opposite heating side of the thermo-module. When used, the ceramic is contacted with a human body to conduct a thermotherapy. Because of such the structure as above, the semiconductor laser can be cooled forcibly and therefore, reliability and endurance are enhanced. (See FIG. 1 and paragraphs 0005, 0009, and 0014 to 0015 of the Patent Document 2.)
With the prior-art optical direct amplifier 110 shown in FIG. 1, however, electric power is necessary not only for driving the exciting light source to be used for exciting the optical amplification medium 111 (i.e., the optical fiber 111b for amplification), for temperature-adjusting the said exciting light source, and for driving a control circuit that controls them but also for driving the heating medium 112. Accordingly, there is a demand that the power consumption of the prior-art optical direct amplifier 110 is lowered with a simple structure at a low cost.